MIDI Velocity stuck intermittently (?)

[Brad Kaenel]

You guys know I'm not afraid to take things apart, so expect a new "Viscount Legend '70s Teardown" thread in a day or so... 

 

Yesterday my C4 key started giving me random max velocity readings. i.e. no matter how lightly or heavily I hit it, it would always sound as if I'd hammered it as loud as possible.  Then, if I left it alone for 20 seconds or so, it would go back to behaving normally, until I intentionally hit it hard again.  Then it would get briefly "stuck" again at max velocity.  Rinse, repeat.

 

This is what I perceive is happening:

 

1) The two velocity sensors on the key start out in their normal positions:

___

*        ___

*         *

*         *

 

2) I hit the key hard, intentionally, producing a high velocity value, and depressing the sensors:

___      ___

*         *

 

3) But on key release, the upper sensor doesn't immediately return to its full resting position:

___       ___

*         *

*         *

 

4) So now, no matter how hard I actually hit the key, both sensors will fire nearly simultaneously, producing a max velocity value

 

5) Eventually, though, the upper sensor pops back up to its full resting position:

___

*        ___

*         *

*         *

 

6) The key will now continue working normally until I intentionally hit it hard, again, and the cycle repeats.

 

Does this sound like a plausible explanation of what I'm experiencing?

 

I've opened up many Fatar-based boards, so I'm not expecting to find anything unfamiliar -- I need to get to the two rubber contact-bubbles beneath the key, and clean out any dust/dirt or other impedance.  What gives me a little bit of a head-scratch though is that in my own experience, dirty contacts have always resulted in the opposite behavior -- no sound at all, rather than max volume.  Just curious if anyone else has a different take on the problem...

 

 

[Mighty Motif Max]

This happened with the Clavinova I grew up with - when I was a little kid, at one point, someone had dropped some spare change on the keys and a dime or two slipped in between them and down into the action. Being a big console unit we didn’t take it apart, but that is when intermittent max velocities started happening in that range of keys, as the coins migrated.

[Brad Kaenel]

I was successful in repairing the MIDI velocity issue with my Legend '70s board.  Here's the 411...

 

1) Open the case; begin with removing the expansion module beauty strip:

 

 

2) Remove the top panel cover

 

 

3) Look at all the SPACE in here!  You could install a tailgate party cooler...

 

 

4) Disconnect, and remove, the expansion module chassis by detaching these three ribbon connectors...

 

...these two screws (one on each end)...

 

...these four screws (two on each end)...

 

...and unplugging this spade connector

 

5) Disconnect the keybed from the case by unplugging this spade connector...

 

...removing these two nuts to free the ribbon cable support...

 

...and removing these two rows of screws from the underside of the case

 

Although I've photographed the underside of the case by rocking the keyboard up, and sitting it on its rear panel, I don't recommend actually removing the two rows of screws in this orientation.  The better, safer thing to do is to leave the keyboard sitting flat, on its base, but cantilevered out into space, overhanging your work table.  Then you can remove the screws from underneath without risking the keybed sliding around or falling down, and perhaps damaging something.

 

6) With all the connectors and screws removed, you can NOW, carefully, rock the keyboard up onto its rear panel, leaving the keybed behind

 

7) Turn the keybed around (still screwed to its plywood base panel) so you can work with it from the rear

 

 

 

😎 Although I did not realize it at the time, it's not necessary to disconnect the keybed from its plywood base, unless you need access to the weight mechanisms.  Consider this step optional, but if you need to, remove these two rows of screws from the underside.  Again, I recommend you do this with the panel laying down flat, and cantilevered over the edge of your work table.

 

9) With the keybed now fully exposed, you need to remove the key(s) in order to get to the velocity control sensor underneath each one.  Having never seen the guts of a TP100 before, it took me a little while to figure out how to disconnect the keys.  To do it, simply apply some gently pressure (use your thumb) to the plastic blade at the back of the key, and push it forward.

 

10) You will hear and feel a "pop" as the keytop disconnects from the weight mechanism, below.  You can then rock the back of the key slightly upwards, then push it forward a bit more, and pull straight up

 

 

White and black keys work exactly the same way.  You don't have to start from the extreme end -- you can easily remove any key from the middle of the bed.  As you will see in a moment, I needed to remove not only the middle C4 key that was giving me problems, but also a few adjacent keys, and a few from the extreme treble end of the keybed.

 

11) Here is the malfunctioning C4 location with the key removed, and the rubber velocity sensor exposed

 

12) The sensors are built in sets of four, unfortunately, so you can't just remove one.  I removed some additional keys to expose a complete, 4-sensor set.  (Note the debris I found in the E4 sensor.  This is the kind of thing you would be looking for if your key has stopped making sound, or is intermittent.  In my case, this was not currently causing any problems, but it could in the future; I cleaned it out.)  There was no obvious problem with the C4 sensor.

 

13) The rubber sensor pads have little "feet" that poke through holes in the circuit board.  You can pull them up, but be careful not to over-stress the rubber or tear it -- it's very soft and very thin.

 

14) On the underside of the pad, each sensor has at least two (in this case, three) multi-level "buttons" that contact the circuit board when the key above is depressed

 

15) With better light, you can now see that *IS* something abnormal with one of the sensor "buttons"

 

16) The "button" is actually torn.  This breakage is what was preventing the button from immediately returning to its neutral position after releasing the key, and the reason why the velocity sensor would temporarily read as "full volume" until the button gradually worked its way back into its "unpushed" state

 

17) Since I don't have these kinds of spare parts just laying around, I decided to "steal" an identical sensor set from underneath the keys at the extreme treble end of the keyboard.  I had to carefully cut a section out with an X-acto knife.  (If you do this, be careful not to press too hard; there are electrical traces on the circuit board that could be damaged if you cut deeper than the thickness of the rubber.)

 

18) I swapped the damaged sensor set out for the good set, and put the damaged set back under the treble keys.  I felt this was an acceptable stop-gap until I could acquire a new sensor pad strip

 

19) That's it.  Reverse the disassembly process to restore the keyboard back to its original state.  Plug it in, and...

...Voile! Key velocity problem fixed!

 

Just as an experiment, I tried "repairing" the damaged sensor button by squirting just a tiny bit of super-glue into the torn "cup".  I didn't know if it would stiffen the rubber to the point that it wouldn't depress properly, but I would be no worse off either way.  As it turned out, that fix *DID* work, and the high-treble key that inherited that damaged sensor is currently working just fine, as well.  I'm going to replace the sensor when I can get a new part, but until then it's great, and that key doesn't get played much anyway -- certainly not nearly as often, or with as much force, as my middle "C".

 

 

 

 

[Brad Kaenel]

Well, here's a first for me -- needing to re-trace my own steps and follow my own instructions to make the same repair again!

 

I guess I must really *bang* the middle "C" on my Legend '70s, because yesterday it started behaving exactly in the same weird way I described here, a little over a year ago.  Back then, my repair was successful, but slightly improvised and a bit makeshift because I needed the fix urgently, and did not have time to order spare parts.  I received them eventually, of course (all hail the parts department at Syntaur).  But being of the "if it ain't broke, don't fix it" mindset I did not re-open the keyboard, opting to wait and see how long my repair would last -- about a year of heavy banging, apparently. 

 

No need to rehash everything; followed the instructions, repair successful, Bob's your uncle.  But I did snap a couple of extra photos that someone might find handy...

 

 

If you've never seen the Legend '70s optional legs, here's their brilliant execution.  The legs don't attach directly to the board.  Instead, the legs attach to a frame, forming a table-like, free-standing platform; then the keyboard attaches to that.  I find this to be quite handy if I just need to quickly run-off with the board alone, leaving the legs behind; particularly handy today as all I had to do was shift it to my work table.

 

 

 

Here's the dodgy C4 bubble contact again.  Oddly, though, I could not discern any obvious damage or deterioration of the contact -- but I replaced it anyway (and also the one I had cannibalized a year ago, from the top register.)

 

 

 

All is well again.  If this turns out to be an annual maintenance task, I've got it down to a science now! 

 

P.S.

Is this not a perfectly excellent location to mount a GSI Gemini box?  I have a couple of blank Legend '70s expansion panels -- plenty of real estate to house an LED display, a couple of lights, and a big-ish knob.  Hmmm... A fascinating DIY project for the newly-retired...